|HST Primer for Cycle 25|
Definitions (HST Orbits, Orbital Visibility Periods, and Visits)HST GO observing time is counted in terms of orbits. Each 96-minute orbit contains a certain amount of useful time when the target can be observed, called the orbital visibility period. The length and timing of the visibility period depends on the declination of the target and on whether there are any special scheduling constraints. Orbits are grouped into larger units called visits; a visit is a series of one or more exposures on a target, including overheads, that will execute in one or more consecutive orbits.
• Guide star acquisition and re-acquisition of a target is required to ensure that HST can maintain adequate pointing during each orbit. Guide star acquisition is needed for the first orbit in a visit, and re-acquisition is needed for subsequent orbits in that same visit. See Section 3.2 for details on telescope guiding.
3. Determine the orbital visibility period of each target in the proposal (described in Section 6.3). Programs requesting 75 or more orbits in a single cycle must use the visibility periods listed in Table 6.1. For LARGE programs, these visibility periods will be enforced for approved programs in Phase II.
5. Lay out all the exposure and overhead times for the program into visits (described in Section 6.5) and add up the number of orbits from each visit to obtain the total orbit request. Each visit must consist of an integer number of orbits. Partial orbits are not granted.6.1.2 Snapshot ProgramsIn a Phase I Snapshot proposal, the PI specifies a requested number of targets, rather than a requested number of orbits. The exposure times and overhead times for Snapshot observations are calculated in similar fashion as for GO observations. The observations for a Snapshot target, including all overheads and the final data dump, are typically 45 minutes or less. See UIR-2014-001 for guidance on orbit length.